The blanket of a snowpack can conceal many different things. For example, a snowpack can conceal the body of an avalanche victim, avalanche debris indicative of the extent and path of an avalanche, layers of weakness that later may become responsible for the formation of an avalanche, and the amount of water contained in the snowpack, among other things. Information about such things can save lives, be helpful in the recovery of human remains, prevent property damage, and provide important information for water-use planning.
Presently, such information is obtained by local investigations of the volume of a snowpack. For example, individuals search for avalanche victims by thrusting probes into the snowpack. Similarly, individuals dig time-consuming snow pits to look for avalanche-prone layers of weakness in a snowpack to predict avalanche danger. Determinations about avalanche flow paths and volumes occupied by avalanche debris are made in similar ways. The snow/water equivalence of a snowpack requires localized and time-consuming measurements about snowpack densities and thicknesses.
These localized investigations and measurements often need to be repeated over large areas to obtain sufficient, or optimal results. For example, the contours for avalanche debris must be determined over several avalanche cycles to assess where structures may safely be built or to determine where to search for an avalanche victim. The stratigraphy of a snowpack, in terms of layers that may contribute to avalanche formation, varies widely over small distances—such as a meter—due to rapidly varying micro-climates in mountainous terrain. A snow pit in a single location, therefore, will often not uncover the weakest portion of a snowpack responsible for the formation of an avalanche in a particular avalanche track. Changes in stratigraphy also have implications for snow/water equivalence, as do changes in snowpack thickness that arise from wind and any number of additional factors, resulting in the need for many measurements.
The time and resources required to make such investigations and measurements are a problem. Where an avalanche victim is involved, every passing minute reduces the probability of finding the victim alive. The investigations and measurements involved in finding an avalanche victim can be so extensive that it is not uncommon to wait for the spring thaw to recover the victim's remains. In terms of avalanche prediction, the number of snow pits required to assess the stratigraphy of a slope in terms of potential for avalanche formation over the region within which an avalanche may form, makes the actual digging of all the snow pits entirely impractical. Educated guesses must be made based on experience, weather, topology, snowpack history, and a wide array of additional factors. The large areas that must be surveyed and the repeated measurements required to assess the flow patterns and regions occupied by avalanche debris also presently require estimations. More objective, less time-consuming, more efficient, and safer methods for acquiring information from volumes in a snowpack over large areas are needed.
The ability of radar to penetrate a snowpack over a large area and to acquire information about varying electromagnetic and geometric properties within the volume of a snowpack that can be correlated to phenomena of interest, makes radar a likely candidate to meet these needs. Prior art demonstrates the ability to harness the impressive range resolution of frequency modulated radar systems to probe a snowpack. Such radar based investigations can be used to discover a body and to reveal properties such as thickness, density, snow-water equivalency, and particular aspects of snowpack stratigraphy by distinguishing between certain layers in the snowpack. The approaches taken in the prior art, however, can only determine the location of reflections from within the snowpack along an axis defined by the direction of propagation, i.e., the range axis.
For a remotely disposed radar system, however, large areas of a snowpack are included within the beam pattern from the radar system. FIG. 1a depicts a system 100 exemplary of this situation in the prior art. In FIG. 1a, a remotely disposed antenna 102 is orientated to transmit toward a snowpack 104 that reposes in mountainous terrain. The remote location of the antenna 102 results in large ranges to locations in the snowpack 104. The footprint 106 illuminated by the antenna 102 becomes larger and larger as range increases according to Equation 1, as provided in FIG. 1b, where ‘λ’ denotes wavelength, ‘R’ denotes range, and ‘d’ denotes the diameter of a circular antenna aperture 102. As appreciated, according to Equation 1, the footprint 106 increases with increasing range. For a particular range, the footprint 106 in FIG. 1a would actually describe an arched shape. However, for simplification of the illustration, the footprint 106 is depicted in a plane normal to the direction of propagation.
The ability to differentiate locations only with respect to the range axis results in ambiguities about the location from which reflections to the radar system originate from within the beam pattern, despite the fine range resolution. As depicted in FIG. 1a, the footprint 106 includes large portions of the snowpack 104. Although the reflections from the same range will not include reflections from the entire snowpack 104, the reflections from large areas of the snowpack 104 will be combined.
Where information about snowpack stratigraphy is sought, changes in the orientation of layers in the snowpack relative to the range axis are particularly problematic for radar systems solely capable of determining locations with respect to the range axis. On the mountainous slopes on which a snowpack reposes, the orientation of a snowpack relative to a remotely disposed radar system can vary widely. FIG. 2 depicts a system 200 exemplifying this additional complication to the situation in the prior art.
In FIG. 2, a remotely disposed radar 202 transmits to a snowpack 204 that reposes in mountainous terrain. The sloping nature of mountainous terrain greatly changes the relative orientation of the range axis 210 from one location to another as seen in the first expanded view 206 and the second expanded view 208. Additionally, mountainous terrain is rugged, and the surface of the bed on which a snowpack reposes undulates and varies widely from location to location.
In the expanded views 206, 208 of the relative orientations of the range axis 210 to the snowpack layers 214a-212d, the hash marks 121a-121f, disposed along the range axis 210, indicate regions that are distinctly resolvable for the radar 202 with its ultra-high-range resolution. However, even with ultra-high-range resolution, at least three distinct problems arise.
First, where the range axis 210 is close to parallel with the snowpack layers 214a-214d, reflections from adjacent layers 214a-214d in the snowpack 204 become confused and become adulterated. However, where the orientation of the range axis 210 becomes more normal, as in the second expanded view 208, the resolvable regions 212a-212f are better oriented to distinguish reflections relative to adjacent strata/layers 214a-214d. 
Second, the differing orientations of the range axis 210 relative to the snowpack layers 214a-214d in the first 206 and the second 208 expanded views indicate that reflections travel different distances along the range axis 210 from different layers 214a-214d depending on the orientation of the range axis 210, making it difficult to determine the relative location and thicknesses of the layers 214a-214d in the snowpack. In the first expanded view 206, where the range axis 210 is almost parallel, great distances must be traveled before boundaries between layers 214a-214d are traversed, making the snowpack 204 and its layers 214a-214d appear very thick. In the second expanded view 208, where the range axis 210 is almost normal to the snowpack 204, the distances traveled more accurately indicate the actual locations and thicknesses of layers 214a-214d within the snowpack 204.
For reasons discussed with respect to FIG. 1 and FIG. 2, and for additional reasons, the radar systems in the prior art must remain close to a snowpack which they probe for information. Also, radar systems in the prior art must maintain the orientation of their range axis relative to snowpack stratigraphy constant along the contour of the snowpack to determine the location from which reflections originate relative to snowpack stratigraphy. For this reason, radar systems are positioned in the prior art directly on top of the snowpack on a sled or beneath a low-hovering helicopter.
FIG. 3 depicts a system 300 exemplary of additional aspects of the situation in the prior art. An antenna 302 depicted in FIG. 3 transmits electromagnetic energy from a prior-art radar system (not shown) to a snowpack 304 that is disposed close to the antenna 302—directly underneath the antenna 302. The antenna 302 is oriented so that the direction of propagation 306, of the waves it transmits, is substantially normal to the contour of the snowpack 304 and the various layers 310-316 that make up the stratigraphy of the snowpack 304.
Since the antenna 302 is maintained close to the snowpack 304, the size of the footprint 318 allows reflections from different portions of the snowpack 304 to be resolved. Additionally, since the direction of propagation 306 is maintained normal to the snowpack 304, the relative location of layers 310-316 in the snowpack 304 and the thicknesses of those layers 310-316 can be determined by the distances traveled by reflections from those layers 310-316.
Unfortunately, such radar systems 300 lose the principal benefits of radar. Such benefits include the ability to scan large areas remotely. These benefits could be employed in the service of meeting the needs of more-objective, less-time-consuming, more-efficient, and safer approaches to acquiring information from volumes in a snowpack 304 over large areas. A radar system 300 that must be maintained close to the snowpack 304 and maintained so that the orientation of the direction of propagation 306 relative to the snowpack 304 is known, cannot meet these needs.
What are needed are a method, an apparatus, and a system capable of scanning large regions of a snowpack to acquire information from within the snowpack from a distance. Such information should be relevant to addressing questions such as, but not limited to, the location of an avalanche victim, the flow patterns of avalanches, regions occupied by avalanche debris, the stratigraphy of a snowpack as it relates to avalanche formation, and the snow/water equivalence of a snowpack. To achieve these ends, such approaches should be capable of remotely pinpointing the location from which reflections back to the radar system originate in three-dimensional space with high resolution.